The present invention relates to an impact wrench for use in fastening or unfastening bolts or nuts. More particularly, the present invention relates to an impact wrench for such use, which includes a rotor driven by a motor thereby to cause the fasteners to be driven into the fixtures through the motion of the anvil hit by the rotor intermittently.
The known impact wrenches have a hammer portion provided by a rotor connected to a rotating shaft. While the rotor is rotated around the anvil, the hammer portion of the rotor is forced into engagement with the anvil under the action of a cam or any other similar device. The engagement of the hammer portion of the rotor occurs when the rotor rotates at a high speed, and the engagement is finished in a moment. In addition, the hammer portion is subjected to various forces while the same is in such a quick movement. Owing to these factors the fabricating of the rotor, particularly of the hammer portion, involves difficulties, and even if it is fabricated, differences in movement occur from rotor to rotor. Furthermore, as the rotor is partially abraded, it becomes difficult for the hammer portion to fall exactly on the anvil, which results in weak blows or even non-blows. This leads to incomplete fastening of fasteners.
In order to solve the problems mentioned above, the inventor has invented an improved wrench which has no hammer or hammer portion, disclosed in Japanese Laid-Open specification No. 127678/82. In the new type of wrench an anvil is placed inside the cylindrical rotor, such that it is ready to be hit by a projection provided on the inside surface of the rotor while to rotor rotates. The anvil is subjected to a relatively large spring load, against which the anvil is positioned, until the same comes into engagement with the side of the projection. Because of this spring-loaded situation, a frictional loss tends to occur between the anvil and the inside surface of the rotor, which retards its speed. As a result, the impact expected to occur therebetween becomes weak. To improve it a larger torque will be required, which gives a detrimentally large reaction to the operator. In addition, at the moment when the anvil is about to come into engagement with the projection, the anvil is likely to displace wholly or partially from the projection due to the action of the spring. This leads to failure to hit or incomplete hitting.